1. Field of the Invention
The present invention relates to a semiconductor device and a fabrication method thereof, and particularly, to a semiconductor device and a fabrication method thereof permitting an improved breakdown capability with low power dissipation (low on-state resistance, low saturation voltage) for metal oxide semiconductor field effect transistors (MOSFETs) and insulated gate bipolar transistors (IGBTs), additionally affording to realize a speedup of switching for IGBTs.
2. Description of the Related Art
There are vertical MOSFETs and vertical IGBTs proposed as semiconductor devices having a trench structure. They use sidewalls of trench as channel regions, and are easy to be short-channeled. The use of trench sidewalls as channel regions affords to have highly dense channel regions formed with a promising high current density.
For a vertical IGBT with a trench structure, a patent material 1*) has disclosed reducing the value of the on-state voltage.
Disclosed in the patent material 1 is an IGBT including: a p-type drain layer; a high resistivity n-type base layer provided on the p-type drain layer; a p-type base layer formed on a surface of the n-type base layer; n-type source regions formed on surface regions of the p-type base layer; gate electrodes formed inside trenches that intrude, between n-type source regions and through the p-type base layer, into intermediate depths of the n-type base layer, having gate oxide films interposed in between; and p-type contact regions formed on surface regions of the p-type base layer, contacting on n-type source regions, wherein the trenches are provided at preset intervals of 1.5 μm or less.
Patent material 1*): JP 11-274484 A (pp. 9-10, FIG. 1)
IGBTs in the past had, as illustrated in FIG. 1: a first base layer 2 of a first conductivity type, which had high resistivity; a collector layer 14 of a second conductivity type, which was formed on a reverse side of the first base layer 2; a second base layer 16 of the second conductivity type, which was formed on an obverse side of the first base layer 2; an emitter layer 13 of the first conductivity type, which was formed as a set of regions thereof on surface regions of the second base layer 16; gate electrodes 8 formed inside trenches extending in a first direction X and intruding, through the emitter layer 13 and the second base layer 16, into intermediate depths of the first base layer 2, having gate insulating films 6 interposed in between; and a base contact layer 4, which was formed as a set of regions thereof on surface regions of the second base layer 16, contacting on the emitter layer 13, extending in the first direction X, having a higher impurity density than the second base layer 16 of the second conductivity type. Interlayer insulating films 10 were provided on gate electrodes 8.
Though unshown in FIG. 1, a collector electrode was connected to the collector layer 14, and an emitter electrode was connected to the emitter layer 13 and the base contact layer 4.
For IGBTs in the past, for the breakdown capability to be increased, it was typical to form, as illustrated in FIG. 1, the base contact layer 4 as a set of p+ diffusion regions between p regions constituting the second base layer 16. However, for this structure, the second base layer 16 made up by p regions had channel portions thereof too narrow to open enough to conduct large currents, resulting in an increased on-state resistance, leading to an increased saturation voltage, as well.